Low density cellular materials commonly used as cores in structural composite sandwich panels are frequently not optimized for performance and/or cost for a given application. Variations in natural materials, such as balsa wood, require structural designers to assume the minimum properties of the material, and those cores usually have higher than required properties and also often have excessive weight. Additionally, the minimum structural properties of balsa wood sometimes greatly exceed the requirements of many weight sensitive composite applications, for example, the blades of wind turbines used to produce electrical energy, and these excess properties are accompanied by excess weight. Engineered foams such as PVC foams having generally isotropic properties are often over-designed for applications which do not require the same properties in all directions. The present invention provides a means of optimizing performance, weight and cost by combining two or more low density cellular materials to form a core panel having composite properties.